Coated ceramic filler materials

ABSTRACT

Coated ceramic filler materials comprised of ceramic particles, fibers, whiskers, etc. having at least two substantially continuous coatings thereon are provided. The coatings are selected so that the interfacial shear strength between the ceramic filler material and the first coating, between coatings, or between the outer coating and the surrounding matrix material, are not equal so as to permit debonding and pull-out when fracture occurs. The resultant, multi-coated ceramic filler materials may be employed to provide ceramic matrix composites with increased fracture toughness. The ceramic filler materials are designed to be particularly compatible with ceramic matrices formed by directed oxidation of precursor metals.

FIELD OF THE INVENTION

The present invention generally relates to coated ceramic fillermaterials having a plurality of superimposed coatings thereon. Thecoated materials are useful as reinforcing materials in ceramic matrixcomposites to provide improved mechanical properties such as fracturetoughness. The present invention also relates to improved compositeswhich incorporate these materials, and to their methods of manufacture.

BACKGROUND OF THE INVENTION

A ceramic composite is a heterogeneous material or article comprising aceramic matrix and filler such as ceramic particles, fibers or whiskers,which are intimately combined to achieve desired properties. Thesecomposites are produced by such conventional methods as hot pressing,cold pressing and firing, hot isostatic pressing, and the like. However,these composites typically do not exhibit a sufficiently high fracturetoughness to allow for use in very high stress environments such asthose encountered by gas turbine engine blades.

A novel and useful method for producing self-supporting ceramiccomposites by the directed oxidation of a molten precursor metal isdisclosed in copending and Commonly Owned U.S. patent application Ser.No. 819,397, described below in greater detail. However, the processingenvironment is relatively severe, and there is a need, therefore, toprotect certain fillers from the strong oxidation environment. Also,certain fillers may be reduced at least partially by molten metal, andtherefore, it may be desirable to protect the filler from this localreducing environment. Still further, the protective means should beconducive to the metal oxidation process, yet not degrade the propertiesof the resulting composite, and even more desirably provide enhancementto the properties.

It is known in the art that certain types of ceramic fillers serve asreinforcing materials for ceramic composites, and the selection orchoice of fillers can influence the mechanical properties of thecomposite. For example, the fracture toughness of the composite can beincreased by incorporating certain high strength filler materials, suchas fibers or whiskers, into the ceramic matrix. When a fractureinitiates in the matrix, the filler debonds from the matrix and spansthe fracture, thereby resisting or impeding the progress of the fracturethrough the matrix. Upon the application of additional stress, thefracture propagates through the matrix, and the filler begins tofracture in a plane different from that of the matrix, pulling out ofthe matrix and absorbing energy in the process. Pull-out is believed toincrease certain mechanical properties such as work-of-fracture byreleasing the stored elastic strain energy in a controlled mannerthrough friction generated between the material and the surroundingmatrix.

Debonding and pull-out have been achieved in the prior art by applying asuitable coating to the ceramic filler material. The coating is selectedso as to have a lower bonding strength with the surrounding matrix thanthe filler, per se, would have with the matrix. For example, a boronnitride coating on silicon carbide fibers has been found to be useful toenhance pull-out of the fibers. However, the use of boron nitride coatedfibers in composites presents significant processing disadvantages. Forexample, the production of ceramic matrix composites containing boronnitride coated materials requires the use of reducing atmospheres sincea thin layer of boron nitride readily oxidizes at temperatures above800°-900° C. A reducing atmosphere, however, is not compatible with thedirected oxidation of molten precursor metal for fabricating ceramiccomposites. Further, in the directed oxidation process the coatingdesirably is compatible with the molten metal in that the molten metalwets the coated filler under the process conditions, for otherwise theoxidation process and matrix growth may be impeded by the filler.

Also, in order to prevent or minimize filler degradation, certain limitsmay be imposed on the conventional fabrication processes, such as usinglow processing temperatures or short times at processing temperature.For example, certain fillers may react with the matrix of the compositeabove a certain temperature. Coatings have been utilized to overcomedegradation, but as explained above, the coating can limit the choice ofprocessing conditions. In addition, the coating must be compatible withthe filler and with the ceramic matrix.

A need therefore exists to provide coated ceramic filler materials whichare capable of debonding and pull-out from a surrounding ceramic matrix.A further need exists to provide coated ceramic filler materials whichmay be incorporated into the ceramic matrix at elevated temperaturesunder oxidizing conditions to provide composites exhibiting improvedmechanical properties such as increased fracture toughness.

In order to meet one or more of these needs, the prior art shows fillermaterials bearing one or more coatings. Carbon is a useful reinforcingfiller but typically is reactive with the matrix material. It thereforeis well known in the art to provide the carbon fibers with a protectivecoating. U.S. Pat. No. 4,397,901 teaches first coating carbon fiberswith carbon as by chemical vapor deposition, and then with areaction-formed coating of a metallic carbide, oxide, or nitride. Due toa mismatch in thermal expansion between the fiber and the coating, thefiber is capable of moving relative to the coating to relieve stress. Aduplex coating on carbon fibers is taught by U.S. Pat. No. 4,405,685.The coating comprises a first or inner coating of a mixture of carbonand a metal carbide and then an outer coating of a metal carbide. Theouter coatings prevent degradation of the fiber due to reaction ofunprotected fiber with the matrix material, and the inner coatinginhibits the propagation of cracks initiated in the outer layer. U.S.Pat. No. 3,811,920, relating to metal matrix composites, disclosescoated fibers as a reinforcing filler, such as boron filaments having asilicon carbide surface layer and an additional outer coating oftitanium carbide. This reference teaches that the additional coating oftitanium carbide improves oxidation resistance as well as provides adiffusion barrier between the filament and metal matrix.

However, the prior art fails to teach or suggest filler materials with aduplex coating for protection from and compatibility with a molten metalin an oxidizing environment during manufacture of the ceramic matrixcomposite by directed oxidation, and yet in the composite exhibitdebonding and pull-out from the surrounding matrix.

DESCRIPTION OF COMMONLY OWNED PATENT APPLICATIONS

The coated ceramic filler materials of this invention are particularlyapplicable or useful in the production of ceramic composites disclosedand claimed in copending and commonly owned U.S. patent application Ser.No. 819,397, filed Jan. 17, 1986, which is a continuation-in-part ofSer. No. 697,876, filed Feb. 4, 1985 (now abandoned), both in the nameof Marc S. Newkirk et al. and entitled "Composite Ceramic Articles andMethods of Making Same". This copending application discloses a novelmethod for producing a self-supporting ceramic composite by growing anoxidation reaction product from a precursor metal or parent metal into apermeable mass of filler.

The method of growing a ceramic product by an oxidation reaction of aparent metal is disclosed generically in copending commonly owned U.S.patent application Ser. No. 818,943, filed Jan. 15, 1986 as acontinuation-in-part of Ser. No. 776,964, filed Sep. 17, 1985 (nowabandoned), which is a continuation-in-part of Ser. No. 705,787, filedFeb. 26, 1985 (now abandoned), which is a continuation-in-part of Ser.No. 591,392, filed Mar. 16, 1984 (now abandoned), all in the name ofMarc S. Newkirk et al. and entitled "Novel Ceramic Materials and Methodsof Making the Same"; and Ser. No. 822,999, filed Jan. 27, 1986, which isa continuation-in-part of Ser. No. 776,965, filed Sep. 17, 1985 (nowabandoned), which is a continuation-in-part of Ser. No. 747,788, filedJun. 25, 1985 (now abandoned), which is a continuation-in-part of Ser.No. 632,636, filed Jul. 20, 1984 (now abandoned), all in the name ofMarc S. Newkirk et al. and entitled "Methods of Making Self-SupportingCeramic Material".

The entire disclosures of each of the Commonly Owned Patent Applicationsare incorporated herein by reference.

Commonly Owned U.S. patent application Ser. No. 818,943 discloses anovel method for producing a self-supporting ceramic body by oxidationof a parent metal (as defined below) to form an oxidation reactionproduct which then comprises the ceramic body. More specifically, theparent metal is heated to an elevated temperature above its meltingpoint but below the melting point of the oxidation reaction product inorder to form a body of molten parent metal which reacts upon contactwith a vapor-phase oxidant to form an oxidation reaction product. Theoxidation reaction product, or at least a portion thereof which is incontact with and extends between the body of molten parent metal and theoxidant, is maintained at the elevated temperature, and molten metal isdrawn through the polycrystalline oxidation reaction product and towardsthe oxidant, and the transported molten metal forms oxidation reactionproduct upon contact with the oxidant. As the process continues,additional metal is transported through the polycrystalline oxidationreaction product formation thereby continually "growing" a ceramicstructure of interconnected crystallites. Usually, the resulting ceramicbody will contain therein inclusions of nonoxidized constituents of theparent metal drawn through the polycrystalline material and solidifiedtherein as the ceramic body cooled after termination of the growthprocess. As explained in these commonly owned patent applications,resultant novel ceramic materials are produced by the oxidation reactionbetween a parent metal and a vapor phase oxidant, i.e., a vaporized ornormally gaseous material, which provides an oxidizing atmosphere. Inthe case of an oxide as the oxidation reaction product, oxygen or gasmixtures containing oxygen (including air) are suitable oxidants, withair usually being preferred for obvious reasons of economy. However,oxidation is used in its broad sense in the commonly owned patentapplications and in this application, and refers to the loss or sharingof electrons by a metal to an oxidant which may be one or more elementsand/or compounds. Accordingly, elements other than oxygen may serve asthe oxidant. In certain cases, the parent metal may require the presenceof one or more dopants in order to favorably influence or facilitategrowth of the ceramic body, and the dopants are provided as alloyingconstituents of the parent metal. For example, in the case of aluminumas the parent metal and air as the oxidant, dopants such as magnesiumand silicon, to name but two of a larger class of dopant materials, arealloyed with the aluminum alloy utilized as the parent metal.

The aforesaid commonly owned patent application Ser. No. 822,999discloses a further development based on the discovery that appropriategrowth conditions as described above, for parent metals requiringdopants, can be induced by externally applying one or more dopantmaterials to the surface or surfaces of the parent metal, thus avoidingthe necessity of alloying the parent metal with dopant materials, e.g.metals such as magnesium, zinc and silicon, in the case where aluminumis the parent metal and air is the oxidant. External application of alayer of dopant material permits locally inducing metal transportthrough the oxidation reaction product and resulting ceramic growth fromthe parent metal surface or portions thereof which are selectivelydoped. This discovery offers a number of advantages, including theadvantage that ceramic growth can be achieved in one or more selectedareas of the parent metal's surface rather than indiscriminately,thereby making the process more efficiently applied, for example, to thegrowth of the ceramic plates by doping only one surface or only portionsof a surface of a parent metal plate. This improvement invention alsooffers the advantage of being able to cause or promote oxidationreaction product growth in parent metals without the necessity ofalloying the dopant material into the parent metal, thereby renderingthe process feasible, for example, for application to commerciallyavailable metals and alloys which otherwise would not contain or haveappropriately doped compositions.

Thus, the aforesaid commonly owned patent applications describe theproduction of oxidation reaction products readily "grown" to desiredthicknesses heretofore believed to be difficult, if not impossible, toachieve with conventional ceramic processing techniques. The underlyingmetal, when raised to a certain temperature region above its meltingpoint, and in the presence of dopants (if required) is transportedthrough its own otherwise impervious oxidation reaction product, thusexposing fresh metal to the oxidizing environment to thereby yieldfurther oxidation reaction product. In forming a ceramic composite body,as described in the aforesaid commonly owned patent application Ser. No.819,397, the parent metal is placed adjacent a permeable mass of fillermaterial, and the developing oxidation reaction product infiltrates themass of filler material in the direction and towards the oxidant andboundary of the mass. The result of this phenomenon is the progressivedevelopment of an interconnected ceramic matrix, optionally containingsome nonoxidized parent metal constituents distributed throughout thegrowth structure, and an embedded filler.

In producing the ceramic composite, any suitable oxidant may beemployed, whether solid, liquid, or gaseous, or a combination thereof.If a gas or vapor oxidant, i.e. a vapor-phase oxidant, is used thefiller is permeable to the vapor-phase oxidant so that upon exposure ofthe bed of filler to the oxidant, the gas permeates the bed of filler tocontact the molten parent metal therein. When a solid or liquid oxidantis used, it is usually dispersed through a portion of the bed of filleradjacent the parent metal or through the entire bed, typically in theform of particulates admixed with the filler or as coatings on thefiller particles.

Polycrystalline bodies comprising a metal boride are produced inaccordance with commonly owned patent application Ser. No. 837,448,filed Mar. 7, 1986, in the name of Marc S. Newkirk, et al., and entitled"Process for Preparing Self-Supporting Bodies and Products MadeThereby". In accordance with this invention, boron or a reducible metalboride is admixed with a suitable inert filler material, and the moltenparent metal infiltrates and reacts with the boron source. This reactiveinfiltration process produces a boride-containing composite, and therelative amounts of reactants and process conditions may be altered orcontrolled to yield a polycrystalline body containing varying volumepercents of ceramic, metal, reinforcing filler, and/or porosity.

SUMMARY OF THE INVENTION

In accordance with this invention, a coated ceramic filler material,adaptable for use as a reinforcing component in a ceramic matrixcomposite, is provided with a plurality of superimposed coatings. Thefiller or reinforcing material useful for this invention includesmaterials where the length exceeds the diameter, typically in a ratio ofat least about 2:1 and more preferably at least about 3:1, and includessuch filler materials as whiskers, fibers, and stable. The coatingsystem includes a first coating in substantially continuous contact withthe ceramic filler material, and one or more additional or outercoatings superimposed over the underlying coating, and in substantiallycontinuous contact therewith. Zonal junctions are formed between thefiller and first coating, between superimposed coatings, and between theouter coating and the ceramic matrix. The coatings are selected so thatthe interfacial shear strength of at least one of these several zones isweak relative to the other zones. As used herein and in the appendedclaims, a zonal junction is not limited to an interface, per se, betweenthe surfaces but also includes regions of the coatings in proximity tothe interfaces, and shear, therefore, is zonal in that it may occur atan interface or within a coating. Further, it is understood that thezonal junction between adjacent surfaces may be minimal or negligibleand exhibit essentially no bonding or adhesion, or the adjacent surfacesmay exhibit appreciable bonding or a strong bond. Upon the applicationof fracture stress to the composite, the weak zone allows for debondingof the filler before the filler fractures, and pull-out or shear of thefiller upon fracture of the filler. This debonding and friction pull-outenhances certain mechanical properties of the composite, and inparticular debonding improves the fracture toughness. Thus, in a duplexcoating system, for example, having a first coating and a second, outercoating superimposed on the first coating, the coatings are chosen tofacilitate debonding and pull-out such that junction between one of thethree interfaces (i.e. the interface between the filler and the innercoating, the interface between the inner coating and the outer coating,the interface between the outer coating and the surrounding matrix, orthe strength of a coating) is weak relative to the other zonal junctionsand allows for debonding and pull-out.

By reason of this invention, the coated ceramic filler materials notonly provide improved mechanical properties, but also the filler isprotected from severe oxidizing environments and yet amenable to theprocessing conditions for making a composite in accordance with theCommonly Owned Patent Applications. Certain fillers are at leastpartially reduced by the molten metal upon contact with the filler, andthe coating protects the filler against this local reducing environment.Thus, the coated fillers are adaptable for use as a reinforcingcomponent in a ceramic matrix composite formed by the directed oxidationreaction of a molten precursor metal or parent metal with an oxidant.Accordingly, a lay-up comprising a parent metal and an adjacent mass offiller is heated in an oxidizing environment to a temperature above themelting point of the metal but below the melting point of its oxidationreaction product which reacts with the oxidant (e.g. air) to form apolycrystalline oxidation reaction product. The oxidation reaction iscontinued thereby growing an oxidation reaction product of increasingthickness which progressively infiltrates the permeable mass of fillermaterial to form the composite product. As explained above, it isdesirable to provide the filler material with two or more superimposedcoatings so as to prolong the useful life or performance of thecomponents and the composite. The filler material is first provided withan inner coating in substantially continuous contact with the fillermaterial which may serve to protect the filler. An outer coating, insubstantially continuous contact with the underlying coating, ispreferably selected so as to be wettable by molten parent metal underthe conditions of the matrix formation process and substantiallynonreactive therewith, and inhibits degradation of the filler materialand the first or inner coating by molten metal and/or the oxidant.Further, the interfacial shear strength of one of the zonal junctions isweak relative to the others thereby permitting debonding and pull-out ofthe filler material on application of stress.

The choice of parent metal and oxidant will determine the composition ofthe polycrystalline matrix, as explained in the commonly owned patentapplication. Thus a filler bearing the coating system may have admixedtherewith a solid or liquid oxidant, such as boron, silica, or lowmelting glasses, or the oxidant may be gaseous, such as anoxygen-containing gas (e.g. air) or a nitrogen-containing gas (e.g.forming gas typically comprising, by volume, 96% nitrogen and 4%hydrogen).

The coated ceramic filler materials of the invention may be utilized inthe manufacture of ceramic matrix composites that provide improvedmechanical properties, especially increased fracture toughness. When soemployed, the thickness of the coatings is sufficient to protect theceramic filler material against corrosive environments such as those ofmolten metals. However, the coatings should not be so thick as to serveas a source of structural defects or to interfere with the function ofthe filler.

The ceramic matrix composites of the present invention are adaptable tofinishing operations such as machining, polishing, grinding, etc. Theresultant composites are intended to include, without limitation,industrial, structural, and technical ceramic bodies for applicationswhere improved strength, toughness and wear resistance are important orbeneficial.

The following terms, as used herein and in the claims, have the statedmeanings as defined below:

The term "oxidation reaction product" means one or more metals in anyoxidized state wherein the metal(s) have given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of the reaction of one or more metals (e.g.aluminum parent metal) with an oxidant such as oxygen or air, nitrogen,a halogen, sulfur, phosphorous, arsenic, carbon, boron, selenium,tellurium; compounds such as silica (as a source of oxygen), andmethane, ethane, propane, acetylene, ethylene, and propylene (as asource of carbon); and mixtures such as H₂ /H₂ O and CO/CO₂ which areuseful in reducing the oxygen activity of the environment.

The term "oxidant" means one or more suitable electron acceptors orelectron sharers and may be a solid, liquid, or gas (vapor) or somecombination of these. Thus, oxygen (including air) is a suitablevapor-phase gaseous oxidant, with air being preferred for reasons ofeconomy. Boron, boron carbide and carbon are examples of solid oxidantsunder this definition.

The term "parent metal" as used in the specification and appended claimsrefers to that metal, e.g. aluminum, which is the precursor of apolycrystalline oxidation reaction product such as alumina, and includesthat metal or a relatively pure metal, a commercially available metalhaving impurities and/or alloying constituents therein, and an alloy inwhich that metal precursor is the major constituent; and when aspecified metal is mentioned as the parent metal, e.g. aluminum, themetal identified should be read with this definition in mind unlessindicated otherwise by the context.

The term "ceramic", as used in this specification and the appendedclaims, is not limited to a ceramic body in the classical sense, thatis, in the sense that it consists entirely of non-metallic, inorganicmaterials, but rather, it refers to a body which is predominantlyceramic with respect to either composition or dominant properties,although the body may contain substantial amounts of one or moremetallic constituents such as derived from the parent metal, mosttypically within a range of from about 1-40% by volume, but may includestill more metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron micrograph taken at 150× magnification ofa coated ceramic filler material in a ceramic matrix and made accordingto the invention.

FIG. 2 is a scanning electron micrograph taken at 850× magnification ofceramic matrix composite having a coated Nicalon® ceramic fiber asfiller material and made according to the Example below.

FIG. 3 is a scanning electron micrograph taken at 250× magnification ofa fractured surface of the composite made with the coated fibersaccording to the Example below showing extensive pull-out of the fibers.

FIG. 4 is a scanning electron micrograph taken at 800× magnification ofa fractured surface of the composite made with uncoated fibers accordingto the Example below showing no pull-out of the fibers.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, coated ceramic fillermaterials are produced by applying a plurality of superimposed coatingsto the ceramic material. Suitable ceramic filler materials which may beused in the invention include metal oxides, borides, carbides, nitrides,silicides, and mixtures or combinations thereof, and may be relativelypure or contain one or more impurities or additional phases, includingcomposites of these materials. The metal oxides include, for example,alumina, magnesia, ceria, hafnia, lanthanum oxide, neodymium oxide,samaria, praseodymium oxide, thoria, urania, yttria, and zirconia. Inaddition, a large number of binary, ternary, and higher order metalliccompounds such as magnesium-aluminate spinel, silicon aluminumoxynitride, borosilicate glasses, and barium titanate are useful asrefractory fillers. Additional ceramic filler materials may include, forexample, silicon carbide, silica, boron carbide, titanium carbide,zirconium carbide, boron nitride, silicon nitride, aluminum nitride,titanium nitride, zirconium nitride, zirconium boride, titaniumdiboride, aluminum dodecaboride, and such materials as Si-C-O-Ncompounds, including composites of these materials. The ceramic fillermay be in any of a number of forms, shapes or sizes depending largely onthe matrix material, the geometry of the composite product, and thedesired properties sought for the end product, and most typically are inthe form of whiskers and fibers. The fibers can be discontinuous (inchopped form as staple) or in the form of a single continuous filamentor as continuous multifilament tows. They also can be in the form oftwo- or three-dimensional woven continuous fiber mats or structures.Further, the ceramic mass may be homogeneous or heterogeneous.

The filler material, useful as a reinforcing or strengthening componentin a ceramic matrix composite, is provided with two or more coatings.The first or inner coating is applied to the filler as a continuous filmor layer, and preferably forms a bond with the filler. The second andany subsequent coatings are superimposed over an underlying coating andbecome attached or bonded therewith as additional layers or stratum.Each coating is applied as a substantially continuous layer, and each isin substantially continuous contact with the underlying coating orfiller in the case of the first coating. The bond formed betweenadjacent surfaces may be weak or negligible in that there may be littleor no adhesion or connection, but in the preferred embodiment there is ameasurable or appreciable bonding or union between surfaces.

In a preferred embodiment of the invention, two coatings only areapplied to the filler material. In such a system utilizing a duplexcoating, the coatings are selected to provide adequate mismatch inbonding strengths so as to allow for debonding and pull-out uponapplication of stress. Also, the duplex coating is selected to provideprotection against degradation of the filler, and the outer coating isselected to exhibit wettability of molten parent metal and to protectthe inner coating from degradation or corrosion in high temperature,oxidizing environments under the conditions of the matrix formationprocess. Also, a system using two coatings rather than three or more, isadvantageous from an economic standpoint.

Thus, the coatings are selected so as to be compatible with the fillermaterial, and to the process conditions for the manufacture of thecomposites. Also, the coatings should complement each other in achievingthe desired characteristics or properties. In a ceramic composite systemhaving incorporated therein a filler with a duplex coating, for example,the first and outer coatings are selected to provide an adequatemismatch in interfacial shear strength so that one of the three zonaljunctions is weak relative to the remaining zonal junctions to providerelative movement between the inner coating and the filler, or betweencoatings, or between the outer coating and the adjacent ceramic matrix.In this manner, debonding and pull-out will occur, thereby improving orenhancing the fracture toughness of the ceramic composite body.

Debonding and pull-out is especially beneficial for filler materialshaving a relatively high length to diameter ratio, such as fibers,typically at least about 2:1 and more particularly at least 3:1. Fillermaterial with a low length to diameter ratio such as particles orspheres, characteristically exhibits crack deflection toughening.

In applying the coatings to the filler material, the thickness of eachcoating and the cumulative thickness of all coatings can vary over awide range. This thickness can depend on such factors as the compositionof each coating and their interaction, the type and geometry of thefiller, and the process conditions and precursor metal used in themanufacture of the composite. Generally, the cumulative thickness forthe coatings should be sufficient to completely cover the ceramic fillermaterial and protect it from oxidation degradation, attack from moltenmetal, and other corrosive environments which may be encountered inemployment of the finished composite. In the preferred embodiment, theinner coating is compatible with the filler material so as not todegrade its integrity, and further the inner coating can be selected toallow for debonding and pull-out or shear. The coating system isselected to be compatible with the matrix material, especially theprecursor for the matrix, and further the coating system is selected soas to be capable of withstanding the process conditions used in themanufacture of the composites. While the inner coating may affordadequate protection against degradation of the filler or allow for shearbetween this first coating and the filler, a second or outer coating isselected to be compatible with the process conditions employed in themanufacture of the ceramic composite body, in that it should besubstantially inert and not degrade, and further should exhibitwettability to molten parent metal when serving as a precursor to theceramic matrix. Also, if the first coating or fiber is susceptible toattack and degradation by the process environment during compositemanufacture or by attack of oxidants diffusing through the matrix duringactual service, the second or outer coating is chosen to protect theinner coating or fiber from exposure to processing conditions and/or enduse conditions. Thus, the coating system protects the fibers fromdegradation, as does one coating superimposed on another, andconcomitantly provides for compatibility for matrix formation and use,and for relative movement to allow for shear. By reason of this coatingsystem, structural degradation of the composite components is mitigatedthereby prolonging the useful life and performance of the composite, andthe fracture toughness of the composite is improved.

If the surface of the filler is very iregular and exhibits nodules,barbs, fibrils, projections, or protuberances, the filler material canmechanically interlock or bond with the adjacent surface including theadjacent coating or adjacent filler material thereby impeding orpreventing debonding and pull-out, which can be deleterious to theproperties of the composite. It therefore is desirable to provide acoating system which is sufficiently thick to completely cover theirregularities in the filler.

The thickness and properties of the coatings may vary depending on thedeposition process and the filler material. In a duplex coating system,the thickness for each coating, in terms of the radius, typically mayrange from about 0.05 to about 25 microns, preferably to about 10microns, but the innermost coating can be monoatomic in order toseparate the second coating from the filler particle. The cumulativethickness for a coating system may be to about 25 microns, and morepreferably 2-10 microns. Usually a coating system having a thicknesswithin this range can be applied to the filler by conventional or knownmeans and will provide the desired properties described above.

It has been found that a number of coating compositions can be employedin the coating system of this invention. These compositions include themetal oxides, nitrides, borides and carbides, alkaline metal salts,alkaline earth metal salts, carbon, silicon, and the like. The choice ofcoating compositions will depend on the filler material, thecompatibility of coatings to each other, and the process conditions forthe manufacture of the ceramic composite. For example, silicon carbidefibers can be used as filler in composites made according to the processdescribed in the Commonly Owned Patent Application. In order to providefor debonding and pullout, the silicon carbide fibers may be coated withboron nitride which prevents a relatively strong bond between the coatedfiber and the surrounding matrix. However, boron nitride may be degradedby the oxidation reaction conditions of the process for making thecomposite. Further, boron nitride may not be wet by certain metals, suchas aluminum, under the conditions of the matrix formation process, andtherefore as an outer coating would tend to interfere with the matrixformation. However, an inner coating exhibiting little or no wettabilityby parent metal under process conditions can be advantageous. Forexample, the coating system may have pores or flaws, but the contactangle of the molten parent metal with the inner coating may precludetransport of the parent metal through any pores or flaws in the innercoating and thereby yet protect the filler from attack by molten metal.The presence of an additional wettable outer coating on the filler wouldthen avoid impedance to the matrix formation process. Therefore, asuitable outer coating such as silicon carbide is applied to the boronnitride coating to achieve compatibility with the forming process and toprotect the boron nitride from degradation, such as by oxidation.Silicon carbide is, for example, wet by doped aluminum and relativelyoxidation-resistant in an air environment at 1000° C., where boronnitride is typically not wet by aluminum, and is oxidation-prone, atthese temperatures. Further, the bond between the two coatings is weakrelative to the other bonds thereby facilitating debonding and pull-outof the fibers during fracture. Other useful coating compositionsinclude, for example, titanium carbide, silicon, calcium silicate,calcium sulfate, and carbon as the inner coating, and silicon, silica,alumina, zirconia, zirconium nitride, titanium nitride, aluminumnitride, and silicon nitride as an outer coating. Other suitablecompositions for the first and outer coatings may be selected for usewith the ceramic filler material provided these coatings complement eachother as in the manner described above.

A typical cross-sectional representation of the coated ceramic fillermaterial is shown in FIG. 1 (discussed below in greater detail). In thistypical example, the ceramic filler material comprising silicon carbidebears a first inner coating of boron nitride and an additional outercoating of silicon carbide. One or more additional outer coatings may beprovided depending on the need. For example, an additional outer coatingof titanium carbide may be applied to the outer coating of siliconcarbide.

The first and outer coatings are deposited onto the ceramic fillermaterial by conventional or known means such as chemical vapordeposition, plasma spraying, physical vapor deposition, platingtechniques, sputtering or sol-gel processing. Achievement of asubstantially uniform coating system according to these prior arttechniques is within the level of skill in this art. For example,chemical vapor deposition of a uniform coating of boron nitride onceramic filler materials can be achieved by using boron trifluoride andammonia at a temperature of about 1000°-1500° C. and a reduced pressureof 1-100 torr; boron trichloride and ammonia at a temperature of600°-1200° C. and reduced pressure of 1-100 torr; borazine at atemperature of 300°-650° C. and a reduced pressure of 0.1-1 torr; ordiborane and ammonia at a temperature of 600°-1250° C. and a reducedpressure of 0.1-1 torr. A coating of silicon carbide by chemical vapordeposition can be accomplished, for example, by usingmethyltrichlorosilane at a temperature of 800°-1500° C. and a pressureof 1-760 torr; dimethyldichlorosilane at a temperature of 600°-1300° C.and a reduced pressure of 1-100 torr; and silicon tetrachloride andmethane at a temperature of 900°-1400° C. and a reduced pressure of1-100 torr.

It should be understood that various combinations of ceramic materialswith first and outer coatings may be produced depending on the specificproperties desired in the coated ceramic material and its ultimateapplication. A possible combination includes silicon carbide fiber witha first layer of titanium carbide and an additional outer layer ofsilicon nitride. Another coating system includes silicon carbide fiberwith a first coating of boron nitride and additional outer coatings ofsilicon carbide and alumina.

The coated ceramic materials employed in the ceramic matrix compositesof the invention are chosen so that debonding and pull-out may beachieved. Thus, the coated fibers are chosen so that the interfacialshear strength between the ceramic filler material and the first coatingis sufficiently different from the interfacial shear strength betweenthe first coating and the additional outer coating or between theoutermost coating and the ceramic matrix to permit relative movementbetween the surfaces and allow for debonding and pull-out.

In the manufacture of ceramic matrix composites according to theinvention, the coated materials may be provided in the form of a loosemass or may be laid up into a porous preform of any desiredconfiguration. The parent metal is placed adjacent the preform. Theparent metal is then heated in the presence of an oxidant to above itsmelting point whereby the molten metal oxidizes to form and develop anoxidation reaction product embedding the coated ceramic material. Duringgrowth of the oxidation reaction product, the molten parent metal istransported through its own otherwise impervious oxidation reactionproduct, thus exposing free metal to the oxidizing atmosphere to yieldadditional reaction product. The result of this process is theprogressive growth of an interconnected ceramic oxidation reactionproduct which optionally may contain nonoxidized parent metal.

A variety of ceramic matrices may be produced by the oxidation reactionof parent metals depending upon the choice of parent metal and oxidant.For example, ceramic matrices may include oxides, nitrides, borides, orcarbides of such parent metals as aluminum, titanium, tin, zirconium orhafnium. The ceramic matrix composites of the invention may comprise, byvolume, 5 to 85% of the coated ceramic filler materials and 95 to 15% ofceramic matrix. A useful composite comprises an alumina matrix formed bythe oxidation reaction of aluminum parent metal in air, or an aluminumnitride matrix by oxidation reaction (i.e., nitridation) of aluminum innitrogen, and incorporating as a reinforcing filler such materials asalumina, silicon carbide, silicon nitride, etc., bearing the coatingsystem. Another useful composite comprises an aluminum boride matrixformed by the reactive infiltration of a bed comprising a boron source(e.g. boron or a reducible metal boride) and a reinforcing fillerbearing the coating system.

The following example illustrates certain aspects and advantages of theinvention.

Two fiber-reinforced alumina-matrix ceramic composite bodies werefabricated in accordance with the present invention. The fibers employedwere Nicalon® ceramic grade silicon carbide as Si-C-O-N (from NipponCarbon Co., Ltd., Japan) measuring approximately 2 inches long andapproximately 10-20 μm in diameter. Each fiber was coated via chemicalvapor deposition with a duplex coating. The duplex coating comprised a0.2-0.5 μm thick first coating of boron nitride applied directly to thefiber, and a 1.5-2.0 μm thick second (outer) coating of silicon carbideapplied to the boron nitride coating.

The duplex coated fibers were gathered into bundles, each containing 500fibers tied with a single fiber tow. Two, 2 inch square by 1/2 inchthick bars of aluminum alloy designated 380.1 (from Belmont Metals,having a nominally identified composition by weight of 8-8.5% Si, 2-3%Zn, and 0.1% Mg as active dopants, and 3.5% Cu as well as Fe, Mn, andNi, but the actual Mg content was sometimes higher as in the range of0.17-0.18%) were placed into a bed of Wollastonite (a mineral calciumsilicate, FP grade, from Nyco, Inc.) contained in a refractory cruciblesuch that a 2 inch square face of each bar was exposed to the atmosphereand substantially flush with the bed, while the remainder of each barwas submerged beneath the surface of the bed. A thin layer of silicasand was dispersed over the exposed surface of each bar to serve as anadditional dopant. Three of the above-described bundles of duplex-coatedfibers were placed on top of each of the two sand-layered metalsurfaces, and these set-ups were covered with Wollastonite.

The crucible with its contents was placed in a furnace which wassupplied with oxygen at a flow rate of 500 cc/min. The furnacetemperature was raised to 1000° C. at a rate of 200° C./hour, and heldat 1000° C. for 54 hours.

The crucible was then removed while the furnace temperature was at 1000°C., and allowed to cool to room temperature. The ceramic compositeproducts were recovered. Examination of the two ceramic compositeproducts showed that an alumina ceramic matrix, resulting from oxidationof aluminum, had infiltrated and embedded the fiber bundles.

Two specimens were machined from each of the two ceramic compositeproducts. FIGS. 1 and 2 are scanning electron micrographs at 150×magnification and 850× magnification, respectively, showing this ceramicmatrix composite. Referring to the micrographs, there is shown thealumina matrix 2 incorporating silicon carbon fibers 4 bearing a firstinner coating 6 of boron nitride and an outer coating 8 of siliconcarbide. One machined specimen from each composite product was testedfor flexural strength (Sintech strength testing machine, Model CITS2000, from Systems Integrated Technology Inc., Stoughton, MA) in 4 pointbend with a 12.67 mm upper span and a 28.55 mm lower span. The valuesobtained were 448 and 279 MPa. The remaining specimen from each productwas tested for Chevron notch fracture toughness, and the values obtainedwere 19 and 17 MPam^(1/2), respectively. FIG. 3 is a scanning electronmicrograph at 250× magnification of the fractured surface of the ceramiccomposite showing extensive pullout of the fibers.

This run was repeated with the exception that the Nicalon® fibers werenot coated. FIG. 4 is a scanning electron micrograph at 800×magnification of the fractured surface showing essentially no pull-outof the fibers. Typical values for strength ranged from 100-230 MPa, andfor toughness ranged from 5-6 MPam^(1/2).

The utility of coated filler material made according to the invention isclearly demonstrated by the Example and the comparative data.

What is claimed is:
 1. A coated ceramic filler material adapted for useas a reinforcing component in a composite comprising a ceramic matrixformed by the directed oxidation reaction of a molten precursor metalwith an oxidant and embedding said filler material, said ceramic fillermaterial having a plurality of superimposed coatings comprising a firstcoating in substantially continuous contact with said filler materialforming a first zonal junction between said filler material and saidfirst coating, and an outer coating in substantially continuous contactwith the underlying coating forming a second zonal junction betweensuperimposed coatings and a third zonal junction between the outermostcoating and the ceramic matrix, and the zonal shear strength of at leastone of the zonal junctions being weak relative to a remainder of theother zonal junctions to permit (1) debonding of said filler material onapplication of stress prior to fracture of said filler material and (2)pull-out of said filler material upon fracture of said filler material.2. The coated ceramic filler of claim 1, wherein said outermost coatingis wettable by, and substantially non-reactive with, said precursormetal in forming said ceramic matrix by said directed oxidationreaction.
 3. The coated ceramic filler of claim 1 or claim 2, whereinsaid outermost coating protects said first coating and said fillermaterial from degradation during the formation of said ceramic matrix.4. The coated ceramic filler material of claim 1 or claim 2, wherein thezonal junction between said ceramic filler and said first coatingcomprises said relatively weak zonal junction having a shear strengthwhich permits debonding and pull-out.
 5. The coated ceramic fillermaterial of claim 1 or claim 2, wherein the zonal junction between theouter coating and the ceramic matrix comprises said relatively weakzonal junction having a shear strength which permits debonding andpull-out.
 6. The coated ceramic filler material of claim 1 or claim 2,wherein the zonal junction between coatings comprises said relativelyweak zonal junction having a shear strength which permits debonding andpull-out.
 7. The coated ceramic filler material of claim 1 or claim 2,wherein said ceramic filler comprises a material selected from the groupconsisting of silicon carbide, Si-C-O-N compounds, alumina, boroncarbide, mullite, zirconia, borosilicate glasses, silicon nitride,silica, titanium nitride, aluminum nitride, and boron nitride, saidfirst coating comprises a material selected from the group consisting ofboron nitride, titanium carbide, silicon, calcium silicate, calciumsulfate and carbon, and said outer coating comprises a material selectedfrom the group consisting of silicon carbide, silicon, silica, alumina,zirconia, silicon nitride, zirconium nitride, titanium nitride, andaluminum nitride.
 8. The coated filler material of claim 2, wherein saidoutermost coating is substantially nonreactive in an oxidizingatmosphere with molten metals selected from the group consisting ofaluminum, magnesium, titanium, zirconium, tin, silicon, and alloysthereof.
 9. The coated ceramic filler material of claim 1 or claim 2,wherein at least one of said coatings is sufficiently thick tosubstantially cover said ceramic filler material to provide a surfacewhich is sufficiently uniform to prevent substantial mechanical bondingof said ceramic filler material with an adjacent surface.
 10. The coatedceramic filler material of claim 1 or claim 2, wherein said coatings areeach about 0.05 to 5 microns in thickness, and the cumulative thicknessof said coatings on said ceramic filler material is no more than about10 microns.
 11. The coated ceramic filler material of claim 1 or claim2, wherein said first coating inhibits the propagation of cracksinitiated at the outer coating from reaching the ceramic fillermaterial.
 12. The coated ceramic filler material of claim 1 or claim 2,wherein said ceramic filler material has a first substantiallycontinuous coating comprises boron nitride thereon and a second coatingcomprising silicon carbide superimposed over and substantiallycontinuous with said first coating.
 13. The coated ceramic fillermaterial of claim 1 or claim 2, wherein said filler material comprisinga material selected from the group consisting of whiskers, fibers orstaple.
 14. The coated ceramic filler material of claim 1 or claim 2,wherein said innermost coating is non-wettable by said precursor metalduring the formation of said ceramic matrix.
 15. The coated ceramicfiller material of claim 12, wherein said ceramic filler materialcomprises silicon carbide or Si-C-O-N compounds.
 16. A self-supportingceramic composite comprising a ceramic matrix having a ceramic fillermaterial incorporated therein and adapted for use as a reinforcingcomponent in said composite, wherein said ceramic matrix is formed asthe oxidation reaction product of a molten precursor metal with anoxidant and embeds said filler material, said ceramic filler materialhaving a plurality of superimposed coatings comprising a first coatingin substantially continuous contact with said filler material forming afirst zonal junction between said filler material and said firstcoating, and an outer coating in substantially continuous contact withthe underlying coating forming a second zonal junction betweensuperimposed coatings and a third zonal junction between the outermostcoating and the ceramic matrix, and the zonal shear strength of at leastone of the zonal junctions being weak relative to a remainder of theother zonal junctions to (1) permit debonding of said filler material onapplication of stress prior to fracture of said filler material and (2)pull-out of said filler material upon fracture of said filler material.17. The ceramic composite of claim 16, wherein said outermost coating iswettable by said molten precursor metal during formation of said ceramicmatrix.
 18. The ceramic composite of claim 16 or claim 17, wherein saidoutermost coating protects said first coating and said filler materialfrom degradation during formation of said ceramic matrix.
 19. Theceramic composite of claim 16 or claim 17, wherein the precursor metalcomprises aluminum and the oxidant comprises air.
 20. The ceramiccomposite of claim 16 or claim 17, wherein said oxidation reactionproduct comprises a metal boride.
 21. The ceramic composite of claim 20,wherein said metal comprises aluminum.